Recent advances in wireless LAN technologies are promoting the growing acceptance of what is known as Hot Spot Service (registered trademark) or similar service offerings designed to allow wireless communication terminals located in public places to acquire information over a network via wireless LAN base stations.
In a wireless LAN environment, a plurality of wireless communication terminals might be set up to obtain the same stream data such as videos and music. In such a case, the stream data generally furnished in large quantities is distributed by so-called IP multicast or like techniques.
The data handled by IP multicast is broadcast on a streaming basis within each wireless LAN cell. The stream data thus distributed is received by a plurality of mobile communication terminals. This setup is intended to reduce the amount of data flowing through each wireless segment of the network.
There already exist techniques (e.g., disclosed in Japanese Translations of PCT for Patent No. 2002-521589) for allocating channels for data distribution through IP multicast, and techniques (e.g., Japanese Patent Laid-open No. Hei 10-308759) for allowing audio-visual equipment in the household to acquire video and audio data from servers on a network through IP multicast. More applications using IP multicast are on their way.
In a wireless LAN environment, as mentioned above, the technique called IP multicast might be used to provide a stream data delivery service over a wireless communication network. In that case, each of the wireless LAN base stations making up the network typically has a limited range of coverage. That means a wireless communication terminal on the go needs to switch its connection from one wireless LAN base station (i.e., access point) to another so that the base station offering the best state of communication is always selected from the location of the terminal on the move.
Automatic selection of a base station by a terminal in transit is generally performed on the basis of the intensity of received radio waves (reception field intensity). The function of automatically selecting a base station relative to a mobile wireless communication terminal (i.e., automatic switching function) is called handover, hand-off, or roaming in its narrow sense.
Illustratively, as shown in FIG. 4, suppose that a base station 203 upon request from a plurality of wireless communication terminals 301 through 305 receives stream data Stm from a server 201 via a router 202 and broadcasts the received stream data Stm simultaneously to the terminals 301 through 305. The terminals 301 through 305 are located in a circular cell covered by the base station 203.
Suppose also that the wireless communication terminal 305 is leaving the cell of the base station 203 delivering the stream data Stm acquired through the router 202, to enter the cell of another base station 204 as shown in FIG. 4. In such a case, the wireless communication terminal 305 switches communication channels to establish communication with the base station 204 that offers a higher level of radio reception than any other base station in the vicinity.
If there is no other wireless communication terminal receiving the stream data Stm in question within the cell of the base station 204, the base station 204 upon request by the wireless communication terminal 305 proceeds to establish a flow of the stream data as indicated by broken lines in FIG. 4.
When the wireless communication terminal getting delivery of the stream data from one base station moves into the cell of another base station, the new base station may or may not be currently offering delivery of the stream data. If the newly reached base station does not currently provide the stream data, it is necessary to establish a new flow of the stream data from the server to that base station and from there to the moving terminal. If that is often the case, the availability of the network as a whole could suffer significantly.
The present invention has been made in view of the above circumstances and provides a wireless communication terminal and a wireless communication method, whereby large quantities of data such as stream data (mass data) are delivered on a wireless communication network with a high degree of overall utilization efficiently.